1. Field of Use
The invention pertains to a novel method of curing concrete mixtures utilizing electrically conductive components placed within the concrete, such as conductive wire or fibers, in a circuit path and using electrical current to create resistive or impedance heating to control the cure of the concrete. This invention pertains to hydraulic cements such as Portland cement, pozzolana (“Roman cement”) and calcium aluminate, as well as resinous cements as heat, moisture and catalytic curing cements, fireclays, kaolin, low alumina clays, and gypsum. Resinous cement materials may be enclosed in hydraulic cement or gypsum covers.
The method taught by this invention can selectively elevate the concrete temperature during setting and hardening to achieve optimum gain of material strength or to accelerate the cure (percentage of hydration) of concrete structures using intra-laminar heat generated by applying electrical energy to electrically conductive members disposed within the structures. The method may also be used in conjunction with concrete containing polymer, wherein the polymer is a catalyst that retards curing until induced by a specified temperature being achieved. This prevents the initiation of auto-accelerated curing in a large mass concrete pour such as in large foundations, footings, retaining walls, etc. The method further incorporates the electrically conductive members as internal reinforcement in the cured, finished structure reducing or obviating the need for reinforcing steel. The heatable and electrically conductive members are provided in various forms including but not limited to pliable forms impregnated with a polymer resin matrix capable of being rigidified during cure and in completely rigidified forms installed prior to the pour of cement mixture. The electrically conductive members may also be affixed to reinforcing steel prior to the concrete pour.
An application of the method of the invention is expedited curing of concrete for use in pre-formed manufactured structures and in tilt-up (“tilt wall”) construction. The invention also pertains to novel concrete products containing the electrically conductive, resistively heatable fibers such as foundation, footing, floor, piers, retaining walls, and slab structures, tilt wall structures, and pre-formed manufactured concrete structures. The electrically conductive fibers can be utilized post cure for heating of the structure or for radiant heating of space. This capability could be used for de-icing of concrete paved surfaces such as roads, walkways, bridges and runways.
2. Background of the Invention
Concrete consists of a mixture of cement, sand, and an aggregate of small stones. When water is added to a dry concrete mix, the cement paste formed should fully coat all sand and aggregate particles, and fill in the void spaces between aggregate particles. The cement paste hardens, owing to the hydration reactions, and bonds the inert sand and aggregate together.
Cement materials such as Portland cement are inexpensive, excellent in durability, fire resistance and other physical properties such as compression strength and stiffness. These materials have been widely used as a building and construction material. However materials made of cement have low tensile strength and impact resistance relative to the compression strength. Concrete materials also have low heat transfer or heat dispersion capabilities. The material also has poor energy absorption. Therefore cement materials are considered to be brittle. Energy absorption, impact resistance and tensile strength are improved by the introduction of reinforcing steel; typically steel rods made from mild steel and commonly termed “rebar”. Mild steel contains 0.08 to 0.15% carbon with tensile strength within the range of 300 to 900 MN/m2.
By way of general background, Portland cement powder is made by firing a mixture of limestone with shale or clay in a rotating kiln. The maximum temperature in the cement kiln is about 950° C. (1742° F.), and at this temperature the lime and clay partially fuse together as a hard clinker. Cement clinker is ground into powder and mixed with a small amount of gypsum (calcium sulfate) to produce dry cement powder. The function of the gypsum is to control the setting characteristics of the cement.
When cement powder is mixed with water a series of complex chemical reactions occurs (“curing”), forming hydrated silicates and aluminates of calcium. (Curing is sometimes referred to as treatment or protection of concrete during the hardening period. However it is used herein as the hydration process.) It is this curing process that causes the wet cement to set and harden as a rigid material. The conditions under which this curing occurs can impact the resultant properties of the cement structure. Some of the hydration reactions take place very slowly and, although the cement will set fairly rapidly, great strength and hardness will not be developed for several days, or weeks, depending on the composition of the cement, the moisture content, and the temperature. The final properties of a concrete will be dependent on a number of factors, including the relative proportions of water, cement, polymer, sand and aggregate in the material, the average size of aggregate particles, the type of aggregate stone used, and the surface texture of the aggregate. The maximum strength of the concrete is achieved upon completion of the cure. (Uncured concrete is sometimes referred to as “green concrete.”)
The speed at which this curing (hydration reaction) occurs is dependent on temperature. At a minimum ambient temperature of 73° F. (23° C.), a waiting period of between five and ten days (120 to 240 hours) must be observed to allow the concrete to attain at least 75% of the design strength (usually 2500-PSI compressive strength). This process is slowed considerably at temperatures below 73° F. resulting in cost increases and scheduling delays.
The moisture content of the cement during curing is important. If there is insufficient water (moisture), full hydration of the cement particles will not occur. For full hydration and the development of maximum strength, a water/cement ratio of about 0.4/1 is necessary. If the water/cement ratio is much in excess of this value the strength of the hardened cement will be reduced.
Recently published studies have reported the effects of temperature on the final compressive strength of cured concrete. If the temperature is either too low or too high, less than optimum compressive strength is achieved. For example in a paper published in 2004 and entitled “Effect of Temperature on the Hydration of Cementitious Materials”, Anton K. Schindler of Auburn University reports that compressive strength for a mortar mixture cured at 50° C. (122° F.) may be 17 percent lower than the compressive strength achieved by the same mortar mixture cured at room temperature (20° C. or 68° F.). This paper adopts work of Kjellsen and Detwiler, 1993. Published papers also state that although increasing concrete temperature during cure speeds the rate of reaction, the hydration reaction does ultimately go to substantial completion regardless of the temperature during curing.
The compressive strengths of plain concrete may be up to 65 MN/m2, in comparison with a compressive strength of about 100 MN/m2 for hardened cement. Other published sources state paving concrete typically has compressive strength of between 3,000 (20.7 MN/m2) and 5,000 psi (34.5 MN/m2). High strength concrete is also defined as having a compressive strength of at least 6,000 psi (41.4 MN/m2) and concrete having compressive strength of 20,000 psi (137.9 MN/m2) have been used in building applications. The strength of concrete in tension, however, is only about one-tenth of the value of compressive strength.
When concrete is subjected to stress (defined as the internal force within a material in balance with an externally applied load), failure probably commences at the interface between aggregate and cement. Aggregate particles with rough surfaces will give concrete of higher strength than will smooth-surfaced aggregate. The tensile strength of concrete (maximum extending load sustained by concrete prior to destruction) is low (up to 5 MN/m2) and, to overcome this disadvantage, concrete fabrications are very often reinforced with steel, typically mild steel in the form of rods (“rebar”). In plain reinforced concrete, a network of steel rods or bars is assembled and the concrete is allowed to set around this framework. The steel reinforcement is positioned in the portion of the concrete member that will be subjected to tensile stresses. For example, in a simply supported beam, the steel lies along the lower portion of the beam. There is a purely mechanical bonding between concrete and steel, and the reinforcement bars are often twisted, or possess surface projections (these may be formed by rolling the bars through patterned rolls) in order to increase the adhesion between steel and concrete.
Another form of reinforced concrete is known as pre-stressed concrete. The concrete is put into a state of compression by means of highly stressed steel wires. When a pre-stressed concrete beam is in service, the initial compressive stresses must be overcome before tensile stresses can be developed within the material. Concrete may be pre-stressed by pre-tensioning, or by post-tensioning. In the former method steel wires are placed in tension before being surrounded by concrete. The externally acting stress on the steel is removed when the concrete has set. In post-tensioning, the concrete is allowed to set and harden around a tube, or tubes. Steel wires are then put through the tubes and these wires are stretched and anchored to the concrete.
The reinforcement of concrete with carbon or other fibers has been studied at least as early as 1994. U.S. Pat. No. 5,308,696 teaches dispersing short carbon fibers (1.0 to 6.0 mm) into an uncured concrete mixture. The concentration of fibers in the mixture may be 20% by volume. U.S. Pat. No. 5,685,902 teaches use of carbon fibers dispersed within a concrete mixture that is cured for 4 weeks (approximately 650 hours) at 20° C. (68° F.). U.S. Pat. No. 6,612,085 teaches use of fibrous composite materials formed in the shape of traditional rebar. The patent claims use of glass and carbon fibers within the “composite rebar”.
The present invention relates generally to a method for expediting the cure of concrete structures while achieving the optimum final compressive strength. An embodiment of the invention relates particularly to the curing of large concrete wall panels used in tilt-up construction practices. One of the advantages of the tilt-up process is the shortened construction times; often only requiring 4-6 weeks for completion. Certain disadvantages exist though in the limitations in geometry due to the inherent properties of concrete reinforced with steel. In tilt-up construction, relatively thin (3.0″-8.0″) wall panels are cast horizontally at ground level and raised into the vertical position by tilting the panel about one end by lifting from the opposite end. The structure is then lifted into a final position to form a structural wall element. Typically, this procedure is performed at the building site with the forms and molding surfaces constructed atop the floor slab, which has first been poured on a prepared sub-grade. With the molding forms positioned, steel reinforcing members are located within the panel area and concrete is poured or pumped into the area defined by the forms. Before the wall elements can be erected, sufficient time must be allowed for the concrete to gain enough strength to withstand the lifting stresses.
Another construction method for producing large, concrete structures involves the casting or molding of the structures at a central location. Once a satisfactory strength level is achieved, the structures are removed from their molds and transported to the erection site. This process does lend itself to a somewhat more controlled environment but does not provide for ideal curing conditions. Large and irregular shaped concrete structures comprising concrete must still observe the basic hydration reaction schedules and are typically less cost effective due to transportation costs. Construction of concrete structures during extreme cold may even become impossible because the water, necessary for cure, can freeze at low ambient temperatures.
In order to accelerate the curing of concrete structures, it has become customary to incorporate additives into the concrete mixture to prevent or retard freezing and alternately provide a heating means so that the concrete will cure more rapidly and thereby facilitate an increase in productivity. Other methods simply employ thermally insulated blankets or covers to contain some of the heat generated naturally by the hydration reaction process. Heating means have historically been through the introduction of steam or pressurized, heated water into an enclosure surrounding the curing forms containing the poured concrete, the use of tubes or conduits that convey a heat transfer medium from a central boiler unit to the surface of the structure or its surrounding mold or form, and even electrically heated molds and forms. See, for example, paragraphs 64 through 85 of the application US 2003/0168164 A1 of Blackmore et al published Sep. 11, 2003.
All of these methods, though addressing the problems with novel and somewhat effective means, are labor intensive and fail to provide an inexpensive, expedient cure mechanism for concrete structures. A heat transfer must still take place from the external heat source through a conveyance apparatus and ultimately through the cross sectional area of the concrete structure in order to accelerate the cure. This process suffers from exorbitant heat loss to the atmosphere. The apparent need exists for a concrete heating method that is energy efficient and economical to implement; ultimately reducing cycle times, labor and finished construction costs. There is also a need for concrete structures that can internally and controllably heated.
It is the principal objective of the present invention to demonstrate an efficient means for controllably introducing heat to concrete structures during cure. It is another object to accelerate the time required to cure concrete. It is another object to provide a heating means that can also provide internal reinforcement to the completed structure. It is another principal objective of the invention to form an improved concrete structure.